1. Field of the Invention
This invention relates to a torque sensor, more particularly to an improved torque sensor capable of noncontactingly detecting drive torque variation as magnetostriction.
2. Description of the Prior Art
In various kinds of rotating drive mechanisms there is a need to measure torque simply and accurately since such measurement is exceedingly useful in a diverse range of industrial fields for analyzing drive mechanisms and for obtaining a better understanding of their operating condition.
Rotary drive mechanisms are used as prime movers in virtually every sector of industry, two of the most common types being automobile engines and industrial motors. In analyzing the operating condition of such mechanisms it is as important to be able to accurately determine torque as it is to determine the number of revolutions. Measurement of torque is particularly important in the case of automobile engines since by measuring the torque at the engine and at the transmission, propeller shaft, differential gear and other points of the power train it is possible to control the ignition timing, the amount of fuel injection, the timing for transmission shift, the gear ratio, etc. As a result of such control, it is possible to improve fuel efficiency, driving characteristics, etc.
Also in the case of industrial motors accurate torque measurement can provide data for optimizing control and diagnosis of rotary drive systems.
Generally, for torque measurement, it is preferable to use a noncontacting sensor so as not to exert an adverse effect on the rotating shaft concerned. Such a sensor it is desirable to be small, light in weight, easy to attach and detach, and easy to maintain.
In the past, a number of different types of torque sensors have been proposed with the aim of satisfying these requirements. These will be explained in the following.
One type is the strain gage sensor which uses a number of strain gages bonded in a bridge arrangement on the surface of the rotating shaft. This system is disadvantageous in that it is troublesome to bond the gages, that it is necessary to provide a telemeter or a slip ring for electrical connection to the strain gages and that it is almost impossible to maintain the sensor mounted on the drive mechanism at all times. As a result the strain gage sensor is normally used only for special instrumentation purposes.
Another type is the twist-angle sensor which uses magnetic elements or light-reflection elements mounted at at least two places on the rotating shaft and measures the torque from the difference in rotational phase between different points on the shaft. Like the strain gage sensor, this type is also large in size and cannot easily be built internally into the drive system.
A third type is the magnetostriction sensor which uses a combination of an excitation coil provided near the rotating shaft and a detection coil and measures variation in torque as magnetostrictive change. Being small and enabling noncontacting measurement, this type of torque sensor can advantageously be built internally into the drive system and promises to meet the various requirements that have not been satisfied by the other types referred to above.
The fact is, however, that conventional magnetostriction sensors generally employ excitation and detection coils that are directly or indirectly wound about the rotating shaft making them large in size and limiting the mounting position. Although it has been proposed to make the excitation and detection coils smaller and mount them around a shaft bearing, this arrangement reduces the amount of magnetic flux passing through the rotating shaft, making it impossible to realize adequate measurement accuracy.